Hydrogeology

Introduction to Hydrogeology

Water is a precious natural resource. Without water there would be no life on Earth. Two-thirds of our body is composed of water by weight.

In regard to water, our everyday lives depend on the following (Hiscock, 2005):

the availability of inexpensive water,

the availability of clean water,

and safe ways to dispose of water after use.

Water supplies are also essential in supporting food production and industrial activity. The most important factor that determine the density and distribution of vegetation is the amount of the precipitation (Fetter, 2001).

Agriculture can flourish in some deserts, but only with water either pumped from the ground or imported from other areas (Fetter, 2001).

Civilizations have flourished with the development of reliable water supplies, and then collapsed as their water supplies failed (Fetter, 2001).

A person requires 3 liters (L) of potable water per day to maintain the essential fluids of the body (Fetter, 2001).

Primitive people in arid lands existed with little more than this amount as their total daily consumption

In New York City the daily per capita water usage exceeds 1000 L; much of this is used for industrial, municipal, and commercial purposes (Fetter, 2001).

The over-exploitation of groundwater by uncontrolled pumping can cause some problems (Hiscock, 2005):

detrimental effects on neighbouring boreholes and wells,

land subsidence,

saline water intrusion,

and the drying out of surface waters and wetlands.

Uncontrolled uses of chemicals and the careless disposal of wastes on land cause groundwater pollution (Hiscock, 2005).

Major sources of groundwater pollution :

agrochemicals,

industrial and municipal wastes,

tailings and process wastewater from mines,

oil field brine pits,

leaking underground storage tanks,

leaking pipelines,

sewage sludge,

and septic systems

Definition and Scope of Hydrogeology

Hydrogeology is the science which mainly deals with the groundwater.

In the broadest sense, hydrology addresses:

occurrence,

distribution,

movement,

and chemistry of all waters of the earth.

Hydrogeology is an interdisciplinary subject and also encompasses aspects of hydrology.

Hydrology is the study of water.

In the broadest sense, hydrology addresses:

occurrence,

distribution,

movement,

and chemistry of all waters of the earth.

A detailed definition of hydrogeology may
be given as follows:

Hydrogeology is the science that studies

occurrence,

distribution,

movement,

physical and chemical properties,

interactions with the environment,

field investigation,

consumption,

conservation

and development

of the water in the rocks of the earth crust.

Hydrogeology
is both a descriptive and an analytic science (Fetter, 2001).

The
development and management of water resources are important parts of hydrogeology
as well.

Field hydrogeology
encompasses:

the methods performed in the field to understand groundwater systems and their connection to surface water sources and sinks.

Hydrogeological investigation is carried
out to understand groundwater system in the field.

Hydrogeological
investigation can
be defined as:

studies,

mapping,

measurement,

analysis,

drilling,

and pumping activities aiming at determination of

occurrence,

reserve,

quality,

consumption,

conservation,

and development

of the groundwater resources.

What is “groundwater”?

Groundwater is defined by water engineers to mean “subsurface water below the water table”.

The
top of the zone of saturation is
called the water
table.

Subsurface water is the sum
of soil moisture (above the water
table) and groundwater (below the
water table).

Who specializes in studying
groundwater and surface waters?

Groundwater tends to be the specialist province of the “hydrogeologist,” while surface waters are the particular domain of the “hydrologist” (Younger,2007).

Often, the groundwater specialists (hydrogeologists) have Bachelor’s degrees in geology or geological engineering, while the surface water specialists (hydrologist) hold degrees in civil engineering or physical geography.

Hydrogeology and Human Affairs

Many of the most important advances in hydrogeology today have been stimulated by studies designed to solve problems of great economic importance (Davis and DeWiest, 1966).This trend will probably continue as the demand for water will undoubtedly increase with growing population and industrialization.Water is used by human in various areas such as municipal, industrial, agricultural, and recreational purposes.An individual in an industrialized urban area may use approximately from four million to twenty million liters of water during his lifetime (Davis and DeWiest, 1966). Water can be used to transport most industrial and domestic wastes at a cost of a few cents per ton.

Unlike most other commodities, water:

can be stored economically in amounts of billions of cubic meters

and retained in storage for many months or even years.

Rapid
increase in
groundwater used for irrigation is largely due to a post-World War
II expansion of irrigated lands.

Groundwater is more desirable than surface water for at least seven reasons (Davis and DeWiest, 1966):

it is commonly free of pathogenic organisms and needs no purification for domestic or industrial uses;

the temperature is nearly constant which is great advantage if the water is used for heat exchange;

turbidity and color are generally absent;

chemical composition is commonly constant;

groundwater storage is generally greater than surface water storage;

radiochemical and biochemical contamination of most groundwater is difficult;

groundwater is available in many areas which do not have dependable surface water supplies

History of Hydrogeology

Water has vital importance for mankind. For this reason the construction of hydraulic structures goes back to very early times. The date of construction of the first hydraulic structure is not known (Usul, 2001).

The oldest water works known are (Usul, 2001):

Irrigation systems built in Mesopotamia, Egypt, Middle
Asia, and China around the famous large rivers of these regions,

Arabian wells,

Persian kanats,

Roman aqueducts,

Kanats were used to collect water from alluvial fan deposits and soft sedimentary rock. These structures were probably used first more than 2500 years ago in Iran. The technique of construction spread rapidly eastward to Afghanistan and westward to Egypt.The foundation of the hydrogeology began in 17th century (Davis and DeWiest, 1966).Pierre Perrault (1608-1680) measured rainfall in the Seine River basin between the years 1668 and 1670. He then estimated runoff from the basin. Edme Mariotte (1620-1684) studied evaporation, infiltration and capillary rise. Edmond Halley (1656-1742) studied evaporation from the Mediterrenean Sea.

Henri
Darcy (1803-1858) was the first person to state clearly the
mathematical law which governs the flow of groundwater (Davis and DeWiest,
1966).

His formula is known as “Darcy’s
Law”.

The development of his formula was the result of experiments with filter sands and was presented in 1856. Darcy’s Law (formula) was presented in a report on the municipal water supply of the city Dijon.

In
19th century

J. Boussinesq, J. Dupuit, P. Forcheimer and A. Theim made important contributions to hydrogeology (Todd and Mays, 2005).Their studies were on the groundwater hydraulics.Improvement of hydrogeolgy also continuedin 20th century.Many scientists have contributed to the development of the hydrogeolgy.Some of these scientists are;

R. Dachler, J. Kozeny, H. Schoeller, and G. Theim.

By the end of 19th century
the scientists in USA played important
roles in the development of the hydrogeology.

About 75% of the water in land areas is locked in glacial ice or is saline.

The
remaining quarter of water in land areas, around 98% is stored underground.

Only a very small amount of
freshwater available to humans and
other biota.

Taking the constant volume of water
in a given reservoir and dividing by the rate of addition (or loss) of water to
(from) it enables the calculation of a residence time for that reservoir.

The
time that a water molecule spends in the ocean and sea more than 4 000 years.

Lakes,
rivers, glaciers and shallow groundwater have residence times ranging
between days and thousands of years.

Groundwater residence
times vary from about 2 weeks to 10 000 years, and longer.

A
similar estimation for rivers provides a value of about 2 weeks.

Basin Properties

Drainage basin

For any cross section on a river, the area above that section which gives all its surface water to this river is called the drainage basin or simply basin for that particular section (Usul, 2001). This area is also named as catchment or watershed

As the control point moves downstream, the basin
becomes larger and larger.

It
has largest or full area when the section is at the point where river reaches a
sea or a lake.

This means that the basin for any cross section along the river is different and therefore each branch of the river has its own subbasin

Basin
is a dynamic and very complex system,
which has mainly two types of
characteristics (Usul, 2001):

Geomorphologic characteristics of the basin are;

area,

shape,

and slope.

These
characteristics can be assumed to be constant, since they change in a very long
time.

Hydrologic characteristics are;

stream shape,

infiltration capacity,

soil conditions,

and vegetal cover

This characteristics change with time.

The line that seperates adjacent
basins, passing through the highest points between them and leaving all
branches (tributaries) of different rivers at opposite sides, is called the water divide or boundary of
the basin.

A suitable scaled topographic
(contour) map of the area is used for this purpose.

The area in this boundary is the drainage area (or catchment area, watershed area).

In some areas surface and subsurface
basin devides may differ depending on the geological features of the area.

In
karst terranes the boundaries of the surface drainage area and
underground watershed in general do not coincide.

The
definition of the boundaries and extention of the watershed area in karst is a
difficult and complex task.

In
order to precisely define it, it is necessary to conduct detailled geological
and hydrogeological investigations .

Basin
properties (after Usul, 2001).

The
longest branch of the river is called the main branch and
its length gives the main channel length (L).

When this branch is continued till the boundary, point C in Figure, the
birds eye view distance between this point and basin outlet is called basin length (LH).

Basin properties (after Usul, 2001).

Basin width (WH) is ratio of the area to the basin
length (WH = A / LH).

The length from the mouth (G) to the centroid, measured on the main
channel is denoted as LC.

The longest basin diameter (LO) is the distance between the outlet (G) and the most distant
point (B) on the basin perimeter

The drainage density is
the total length of the drainage network per unit area of the basin:

Dd= ∑ Lu / A

where Dd is the drainage density

[km/km2]

,

Lu is the total length of the streams
of all orders [km],

and A is the area of the basin [km2].

The drainage frequency (hydrographic density) is the number of flow channels per unit area
of the basin:

Df = ∑ Nu / A

where
Df is the drainage frequency [km-2], Nu is
the number of streams,